rtc: ds1307: add frequency_test_enable attribute on m41txx

On m41txx you can enable open-drain OUT pin to check if offset is ok.
Enabling OUT pin with frequency_test_enable attribute, OUT pin will tick
512 times faster than 1s tick base.

Enable or Disable FT bit on CONTROL register if freq_test is 1 or 0.
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ds1307.c

@@ -1050,6 +1050,94 @@ static int m41txx_rtc_set_offset(struct device *dev, long offset)
 				  ctrl_reg);
 }
 
+static ssize_t frequency_test_enable_store(struct device *dev,
+					   struct device_attribute *attr,
+					   const char *buf, size_t count)
+{
+	struct ds1307 *ds1307 = dev_get_drvdata(dev);
+	bool freq_test_en;
+	int ret;
+
+	ret = kstrtobool(buf, &freq_test_en);
+	if (ret) {
+		dev_err(dev, "Failed to store RTC Frequency Test attribute\n");
+		return ret;
+	}
+
+	regmap_update_bits(ds1307->regmap, M41TXX_REG_CONTROL, M41TXX_BIT_FT,
+			   freq_test_en ? M41TXX_BIT_FT : 0);
+
+	return count;
+}
+
+static ssize_t frequency_test_enable_show(struct device *dev,
+					  struct device_attribute *attr,
+					  char *buf)
+{
+	struct ds1307 *ds1307 = dev_get_drvdata(dev);
+	unsigned int ctrl_reg;
+
+	regmap_read(ds1307->regmap, M41TXX_REG_CONTROL, &ctrl_reg);
+
+	return scnprintf(buf, PAGE_SIZE, (ctrl_reg & M41TXX_BIT_FT) ? "on\n" :
+			"off\n");
+}
+
+static DEVICE_ATTR_RW(frequency_test_enable);
+
+static struct attribute *rtc_freq_test_attrs[] = {
+	&dev_attr_frequency_test_enable.attr,
+	NULL,
+};
+
+static const struct attribute_group rtc_freq_test_attr_group = {
+	.attrs		= rtc_freq_test_attrs,
+};
+
+static void rtc_calib_remove_sysfs_group(void *_dev)
+{
+	struct device *dev = _dev;
+
+	sysfs_remove_group(&dev->kobj, &rtc_freq_test_attr_group);
+}
+
+static int ds1307_add_frequency_test(struct ds1307 *ds1307)
+{
+	int err;
+
+	switch (ds1307->type) {
+	case m41t0:
+	case m41t00:
+	case m41t11:
+		/* Export sysfs entries */
+		err = sysfs_create_group(&(ds1307->dev)->kobj,
+					 &rtc_freq_test_attr_group);
+		if (err) {
+			dev_err(ds1307->dev,
+				"Failed to create sysfs group: %d\n",
+				err);
+			return err;
+		}
+
+		err = devm_add_action_or_reset(ds1307->dev,
+					       rtc_calib_remove_sysfs_group,
+					       ds1307->dev);
+		if (err) {
+			dev_err(ds1307->dev,
+				"Failed to add sysfs cleanup action: %d\n",
+				err);
+			sysfs_remove_group(&(ds1307->dev)->kobj,
+					   &rtc_freq_test_attr_group);
+			return err;
+		}
+		break;
+	default:
+		break;
+	}
+
+	return 0;
+}
+
 /*----------------------------------------------------------------------*/
 
 static int ds1307_nvram_read(void *priv, unsigned int offset, void *val,
@@ -1792,6 +1880,10 @@ static int ds1307_probe(struct i2c_client *client,
 	if (err)
 		return err;
 
+	err = ds1307_add_frequency_test(ds1307);
+	if (err)
+		return err;
+
 	if (chip->nvram_size) {
 		struct nvmem_config nvmem_cfg = {
 			.name = "ds1307_nvram",